1. Field of the Invention
This invention relates to quasi-optic arrays, such as grid arrays, and, in particular to techniques for enhancing the gain and bandwidth of active unit cells that comprise such arrays.
2. Description of Related Art
Broadband communications, radar and other imaging systems require the transmission of radio frequency (xe2x80x9cRFxe2x80x9d) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many applications at these high frequencies, a technique called xe2x80x9cpower combiningxe2x80x9d has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at higher frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.
Several years ago, spatial power combining using xe2x80x9cquasi-opticsxe2x80x9d was proposed as a potential solution to these problems. The theory was that an array of microwave or millimeter-wave solid state sources placed in a resonator could synchronize to the same frequency and phase, and their outputs would combine in free space, minimizing conductor losses. Furthermore, a planar array could be fabricated monolithically and at shorter wavelengths, thereby enabling potentially thousands of devices to be incorporated on a single wafer.
Since then, numerous quasi-optical devices have been developed, including detectors, multipliers, mixers, and phase shifters. These passive devices continue to be the subject of ongoing research. Over the past few years, however, active quasi-optical devices, namely oscillators and amplifiers, have evolved. One benefit of spatial power combining (over other methods) using quasi-optics is that the output power scales linearly with chip area. Thus, the field of active quasi-optics has attracted considerable attention in a short time, and the growth of the field has been explosive.
It is believed that the first quasi-optical grid array amplifier was a grid developed by M. Kim et al at the California Institute of Technology. This grid used 25 MESFET differential pairs, demonstrating a gain of 11 dB at 3 GHz. As shown in FIG. 1, a typical grid amplifier 10 is an array of closely-spaced differential pairs of transistors 14 on an active grid 12 sandwiched between an input and output polarizer, 18, 24. An input signal 16 passes through the horizontally polarized input polarizer 18 and creates an input beam incident from the left that excites rf currents on the horizontally polarized input antennas 20 of the grid 12. These currents drive the inputs of the transistor pair 14 in the differential mode. The output currents are redirected along the grid""s vertically polarized antennas 22, producing a vertically polarized output beam 30 via an output polarizer 24 to the right.
The cross-polarized input and output affords two important advantages. First, it provides good input-output isolation, reducing the potential for spurious feedback oscillations. Second, the amplifier""s input and output circuits can be independently tuned using metal-strip polarizers, which also confine the beam to the forward direction. Numerous grid amplifiers have since been developed and have proven thus far to have great promise for both military and commercial RF applications and particularly for high frequency, broadband systems that require significant output power levels (e.g. greater than 5 watts) in a small, preferably monolithic, package. Moreover, a resonator can be used to provide feedback to couple the active devices to form a high power oscillator.
Unfortunately, conventional active grids arrays, such as amplifiers and oscillators have not been as efficient as is desirable. In particular, reported grid array amplifiers using simple differential pair unit cells exhibit only relatively limited gain, on the order of 10 dB or less. The limited gain limits the applications to which conventional grid arrays may be employed. Moreover, in addition to gain, frequency response and impedance matching are all critical criteria for the design of microwave and millimeter wave devices. The current state of quasi-optic amplifier design does not adequately address these issues.
There is thus a definite need for active quasi-optic grid arrays, and particularly the unit cells that comprise the arrays, that yield higher gains, at higher frequencies. It would be further desirable to have such components that offer greater flexibility in impedance matching, thereby improving the bandwidth and manufacturability of such designs.
The present invention, which addresses these needs, resides in an architecture for improving the gain and bandwidth of active quasi-optic grid array unit cells. A method of the invention includes providing a two active networks and applying reinforcing signals to each of the networks. The first active network is driven by an input signal of a given magnitude and polarity and the second active network is driven by an input signal that is equal and opposite to the input signal that drives the first network. The first network includes a signal input port, a signal output port, a feedback tie-in port and a feedback take-off port. Similarly, the second network includes a signal input port, a signal output port, a feedback tie-in port, and a feedback take-off port. The method then applies to the feedback tie-in port of the first active network, via a feedback path, a reinforcing signal derived from the feedback take-off port on one of the active networks of the unit cell, and applies to the feedback tie-in port of the second active network, via a feedback path, a reinforcing signal derived from the feedback take-off port on the other one of the active networks of the unit cell. Each of the feedback paths includes a substantially identical feedback network having a transfer function that causes the reinforcing signal applied to each network to add constructively to the input signal applied to that network within the frequency range of interest.
In one aspect of the invention, the reinforcing signal applied to the feedback tie-in port of the first network is derived from the feedback take-off port of the second network, and the reinforcing signal applied to the feedback tie-in port of the second network is derived from the feedback take-off port of the first network. This may be referred to as a cross-coupled, regenerative feedback topology.
In a specific implementation of this aspect, the feedback tie-in port of each network is internally connected to the signal input port of that network and the feedback take-off port of each network is internally connected to the signal output port of that network. This embodiment includes a simple differential pair of active device connected using a crossed-coupled, regenerative feedback topology.
In an alternative aspect of the invention, the reinforcing signal applied to the feedback tie-in port of the first network is derived from the feedback take-off port of the first network and the reinforcing signal applied to the feedback tie-in port of the second network is derived from the feedback take-off port of the second network (broad shuntxe2x80x94shunt config.) In this embodiment, the feedback path of each network includes a substantially identical feedback network and the reinforcing signal applied to the feedback tie-in port is derived via a combination of a frequency dependent phase shift from the active network and an additional frequency dependent phase shift from the feedback network. In a more detailed aspect of this xe2x80x9cshuntxe2x80x94shuntxe2x80x9d configuration, the feedback tie-in port of each network is internally connected to the signal input port of that network, and the feedback take-off port of each network is internally connected to the signal output port of that network.
The present invention also discloses a differential unit cell for a quasi-optic grid array. The cell comprises first and second active networks and first and second reinforcing signal paths. Each network has a signal input port, an amplification stage, a signal output port, a feedback take-off port, a feedback tie-in port and a reference port. The networks are connected to each other via the respective reference ports.
In one embodiment, the first reinforcing signal path connects the feedback tie-in port of the first network with the feedback take-off port of the second network, and the second reinforcing signal path connects the feedback tie-in port of the second network with the feedback take-off port of the first network. Each of the two reinforcing signal paths includes a feedback network. In a more detailed aspect of this embodiment, the first active network includes a second amplification stage connected to the first amplification stage via a coupling impedance network and the second active network includes a second amplification stage connected to the first amplification stage via a coupling impedance network. In another embodiment of this differential unit cell, the first reinforcing signal path connects the feedback tie-in port of the first network with the feedback take-off port of the first network, and the second reinforcing signal path connects the feedback tie-in port of the second network with the feedback take-off port of the second network. Each of the two reinforcing signal paths includes a feedback network. In a more detailed aspect of this embodiment of the differential unit cell, the first active network includes a second amplification stage connected to the first amplification stage via a coupling impedance network and the second active network includes a second amplification stage connected to the first amplification stage via a coupling impedance network.
The present invention also discloses yet an even more detailed description of the differential unit cell for a quasi-optic grid array. In this embodiment, the cell has a first input port for an input signal, a second input port for an input signal that is equal and opposite to the input signal at the first input port, a first output port and a second output port. The cell also includes a first three-terminal active device having a control electrode connected to the first input port, an anode connected to the first output port and a cathode and a second three-terminal active device having a control electrode connected to the second input port, an anode connected to the second output port and a cathode connected to the cathode of the first active device in a differential pair configuration. In this single differential pair embodiment, the anode of the first active device is connected to the control electrode of the second active device through a first regenerative feedback network and the anode of the second active device is connected to the control electrode of the first active device through a second regenerative feedback network. This may be referred to as a cross-coupled regenerative feedback differential pair cell.
In yet an another differential pair design with positive feedback., disclosed is a unit cell for a quasi-optic grid array having a first input port for an input signal and a second input port for an input signal that is equal and opposite to the input signal at the first input port, a first output port and a second output port. The cell also includes a first three-terminal active device having a control electrode control connected to the first input port, an anode connected to the first output port and a cathode and a second three-terminal active device having a control electrode control connected to the second input port, an anode connected to the second output port and a cathode connected to the cathode of the first active device. The three terminal device can be any kind of active device, such as a FET or BJT. The anode of the first active device is connected to the control electrode of the first active device through a first regenerative feedback network in a differential pair configuration, and the anode of the second active device is connected to the control electrode of the second active device through a second regenerative feedback network. This may be referred to as a shuntxe2x80x94shunt, regenerative feedback differential pair cell.
Both networks in the unit cell may also include multiple amplification stages to further improve performance. In particular, the first active network may have an input port, an output port and a reference port, and at least a first and second amplification stage. In turn, each stage includes at least one three-terminal active device having a cathode, an anode and a control electrode. Similarly, the second active network, which is substantially identical to the first network, may have an input port, an output port and a reference port, and at least a first and second amplification stage. Each stage of this network also includes at least one three-terminal active device having a cathode, an anode and a control electrode. The second network is differently coupled to the first network via their respective reference ports. (multi-transistor per cell).
This multi-amplification per cell embodiment may be connected in several ways. In one design within the first network, the control electrode of the active device of the first amplification stage is connected to the input port of the network, the anode of the active device of the second amplification stage is connected to the output port of the network and the anode of the active device of the first amplification stage is connected to the control electrode of the active device of the second amplification stage through a coupling impedance network.
Alternatively, within the first network, the control electrode of the first active device is connected to the input port, the cathode of the first active device is connected to the reference port, and the anode of the first device coupled to the cathode of the second device through an impedance network, the control terminal of the second device is connected to a bias voltage through an impedance network, and the anode of the second device is connected to the output port.
As discussed in detail, cell designs that incorporate multiple stages of amplification per network may preferably be designed into the positive feedback implementations disclosed by the present invention. Two such combinations are explicitly disclosed.
In one of the combinations, the unit cell for a quasi-optic grid array comprises a first active network, a second active network, a first reinforcing signal path, and a second reinforcing signal path. The first active network includes a first signal input port for receiving an input signal, a first signal output port, a first feedback take-off port and a first feedback tie-in port. The first network further includes a first three-terminal active device having a control electrode connected to the signal input port, an anode and a cathode, and a second three-terminal active device having a control electrode internally connected to the anode of the first active device via a coupling impedance network, an anode connected to the signal output port, and a cathode connected to the cathode of the first active device.
The second active network includes a second signal input port for receiving an input signal that is equal and opposite to the first input signal, a second signal output port, a second feedback take-off port and a second feedback tie-in port. The second network further includes a third three-terminal active device having a control electrode connected to the second signal input port, an anode and a cathode, and a fourth three-terminal active device having a control electrode internally connected to the anode of the third active device via a coupling impedance network, an anode connected to the second signal output port, and a cathode connected to the cathodes of the first, second and third active devices.
In this design, the first reinforcing signal path connects the second feedback take-off port to the first feedback tie-in port via a first feedback network; and the second reinforcing signal path connects the first feedback take-off port to the second feedback tie-in port via a second feedback network. Moreover, the first feedback take-off port is internally connected to the anode of the first active device, the first feedback tie-in port is internally connected to the control electrode of the first active device, the second feedback take-off port is internally connected to the anode of the second active device, and the second feedback tie-in port is internally connected to the control electrode of the second active device. In other words, the cell comprises two substantially identical two-stage cascade networks differentially connected in a cross-coupled topology.
In another detailed combination, the unit cell for a quasi-optic grid array includes a first active network, a second active network, a first reinforcing signal path, and a second reinforcing signal path. The first active network includes a first signal input port for receiving an input signal, a first signal output port, a first feedback takeoff port and a first feedback tie-in port. It further includes a first three-terminal active device having a control electrode connected to the signal input port, an anode and a cathode, and a second three-terminal active device having a control electrode internally connected to the anode of the first active device via a coupling impedance network, an anode connected to the signal output port, and a cathode connected to the cathode of the first active device.
The second active network includes a second signal input port for receiving an input signal that is equal and opposite to the first input signal, a second signal output port, a second feedback take-off port and a second feedback tie-in port. This second network further includes a third three-terminal active device having a control electrode connected to the second signal input port, an anode and a cathode, and a fourth three-terminal active device having a control electrode internally connected to the anode of the third active device via a coupling impedance network, an anode connected to the second signal output port, and a cathode connected to the cathodes of the first, second and third active devices.
The first reinforcing signal path connects the first feedback take-off port to the first feedback tie-in port via a first feedback network, and the second reinforcing signal path connects the second feedback take-off port to the second feedback tie-in port via a second feedback network. In this specific design, the first feedback takeoff port is internally connected to the anode of the second active device, the first feedback tie-in port is internally connected to the anode of the first active device, the second feedback take-off port is internally connected to the anode of the fourth active device, and the second feedback tie-in port is internally connected to the anode of the third active device. In other words, the cell comprises two substantially identical two-stage cascade networks differentially connected in a shuntxe2x80x94shunt topology.